A polyacetal resin, also referred to as a POM resin, has many excellent properties such as mechanical properties, thermal properties, electrical properties, slidability, formability, and the like, and is widely used mainly as a structural material or a mechanical component and the like in electrical devices, automobile components, precision machinery components, and the like. However, as the fields of application of polyacetal resins have broadened, there has been a tendency for the required properties to become increasingly advanced, numerous, and specialized.
As one example of this, there is a requirement for a material which, while maintaining the inherently excellent formability, surface appearance, and the like of polyacetal resin, has further improved stiffness, surface hardness, sliding properties and the like. In response to such requirements, as a means of increasing the stiffness, a method of filling the polyacetal resin with a fibrous filler is common, but there are problems with this method such as inferior external appearance, a decline in sliding characteristics, and the like, of the molded product.
In order to increase the stiffness, there has been proposed a polyacetal resin wherein 99.9 to 90 parts by weight of (A) a linear polyacetal resin having a melt index of 1 to 50 g/min and obtained by copolymerizing 99.5 to 97.5% by weight of (a) trioxane and 0.5 to 2.5% by weight of (b) a compound selected from a monofunctional cyclic ether compound and a monofunctional cyclic formal compound, is blended with 0.1 to 10 parts by weight of (B) a branched or crosslinked polyacetal resin having a melt index of 0.1 to 10 g/min and obtained by copolymerizing 99.49 to 95.0% by weight of (a) trioxane, 0.5 to 4.0% by weight of (b) a compound selected from a monofunctional cyclic ether compound and monofunctional cyclic formal compound and 0.01 to 1.0% by weight of (c) a polyfunctional glycidyl ether compound with a functional group number of 3 to 4, wherein the (A) linear polyacetal resin and the (B) branched or crosslinked polyacetal resin are selected such that a ratio of the melt index of the (A) linear polyacetal resin and the melt index of the (B) branched or crosslinked polyacetal resin satisfies the relation 0.02 MIB/MIA≦1.5 (where MIA is a melt index of the (A) linear polyacetal resin and MIB is a melt index of the (B) branched or crosslinked polyacetal resin) (for example, refer to Patent Document 1). The polyacetal resin composition disclosed in Patent Document 1 is excellent in having high stiffness, dimensional stability, and creep properties.
Further, in order to increase the sliding properties, there has been proposed a polyacetal resin composition wherein, with respect to 100 parts by weight of (A) a polyacetal resin, there is blended 0.01 to 100 parts by weight of (B) a polyacetal copolymer having a total terminal group amount of 15 to 150 mmol/kg and obtained by copolymerizing 100 parts by weight of (a) trioxane, and 0.0005 to 2 parts by weight of (b) a compound having two or more cyclic ether units in each molecule, and 0 to 20 parts by weight of (c) a compound having one cyclic ether unit in each molecule (for example, refer to Patent Document 2). The polyacetal resin composition disclosed in Patent Document 2 is excellent in stiffness, surface hardness, and sliding properties. Further, the polyacetal resin composition disclosed in Patent Document 2 has suitable toughness because the total terminal group amount of the (B) component is suppressed to be no greater than a fixed amount.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2003-342442    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2002-003694